ANION GAP
The
Anion Gap is an approximate measurement of ions, that
is molecules with a charge, either negative or positive. Sodium and potassium
are positively charged and therefore called cations; this is why they are often
represented as Na+ and K+ respectively. These two are the
main cations measured in the blood. Negatively charged molecules are called
anions, and the major anions measured in the blood are chloride (Cl-)
and bicarbonate (HCO3-). You can tell those electrolytes
in the blood because they are measured as mEq/L rather than in milligrams/dL.
Because your body must remain neutral, or in other words having the same amount
of positive ions as negative ions, the equation (Na+ + K+
+ unmeasured cations) must = (Cl- + HCO3- +
unmeasured anions). Some of the unmeasured cations (~7Mmol/L) include calcium,
magnesium, and most other minerals. Unmeasured anions (~24 Mmol/L) include
proteins like albumin, and phosphates, sulfates, etc. There are always more
unmeasured anions than cations, and thus the "anion gap" equation,
(Na+ + K+) - (Cl- + HCO3-),
is always greater than zero.
If
your lab report does not include an anion gap, you can calculate it yourself by
converting the CO2 result to HCO3- by
subtracting 1 mEq/l from the CO2 content…
Optimum
Value: 10 to 12 MMol/L depending on
the method of calculation.
The
Anion Gap is increased when there are excessive anions/acids in the blood. This
is either from too much acid production or insufficient removal of acids
(either through the lungs, stomach, or kidneys). Excess acids lead to a rapid
respiratory rate (the body wants to blow off the extra CO2), an
inability to hold your breath (the acid build up forces you to exhale), low
blood pressure (due to vasodilation), fatigue, poor appetite, etc. The high
anion gap indicates that the electrical charge of the fluids are too negative
compared to the inside of the cell. Because the charge across cell membranes is
required for many enzymes and energy production, a reduced charge may result in
less energy production (oxidative phosphorylation and ATP). A high anion gap
may also indicate a functional need for alkaline minerals.
The
electrical potential between the inside of the cell and the outside of the cell
is basis for nearly all transactions that occur with in the cell. Within the
cell the chief cation is potassium and the chief anion is phosphorus. Outside
the cell the sodium is balanced by chloride. The balance between the inside and
the outside of the cell is maintained by a pump that sends potassium in and
sends sodium out. When there is an insufficiency of these electrolytes,
electricity can't be generated - as a result energy production and cellular
function is compromised.
Common causes
of an elevated Anion Gap include:
- Ketoacid overproduction due to fat
metabolism (diabetes, alcohol, starvation)
- Lactic Acid overproduction due to
respiratory failure (the tissue has inadequate oxygen), genetic defects of
enzymes of carbohydrate metabolism, nutritional deficiencies that impair
the bodies ability to metabolize lactic acid (B vitamins, especially vitamin
B1)
- Inability to excrete acids (sulfate and
phosphate) due to renal disease (usually with an elevated BUN and
creatinine).
- Dehydration.
- Medications such as salicylates causing a
metabolic block.
- Toxins such as ethylene glycol, methanol,
paraldehyde, propyl alcohol
The Anion Gap
is decreased by free radical pathology due to overproduction of
alkaloids. Other causes that have been reported associated with a reduced anion
gap are
·
Alkalosis
for any reason
- Hyperchloremic acidosis (excess chloride)
- Multiple Myeloma
- Hyponatremia (low blood sodium level; see
appropriate lab finding webpage)
- Hypoalbuminemia (see albumin; can increase the amount of
free blood calcium)
- Bromide Ingestion (displaces chloride)
- Uncalculated blood cations (calcium,
magnesium)
·
Lithium
toxicity (can be due to effects on sodium)
·
Primary
hypothyroidism
·
Kidney
disease (due to the loss of the cations sodium and or potassium)
·
Polymyxin
B
